"After HIV infects a person, it must recruit and latch onto particular human proteins so that the virus can replicate throughout the body," said Zhixiang Chen, a Purdue professor of botany and plant pathology. "We found that cauliflower mosaic virus relies on the same protein complex to multiply in plants."

Cauliflower mosaic virus, known as CaMV, attacks a plant group that includes cauliflower, broccoli, cabbages, turnips, canola and many types of mustard.

"We believe that the proteins in these host plants might be particularly important for these types of viruses, such as HIV, because if you block them, then the viruses simply can't replicate."

The retrovirus HIV and the pararetrovirus CaMV both use reverse transcription to recruit the host's proteins in order to reproduce and spread infection. Transcription in cells is the process in which a gene's DNA code is copied into RNA, which, in turn, carries the information to another part of the cell or to another cell. In reverse transcription, used by viruses such as HIV and CaMV, the virus' RNA is copied into DNA after it latches on to a victim's cell. This allows the virus to easily integrate into the host's genome and then reproduce in other cells.

Chen and his colleagues published a report on their study in the most recent issue of the journal The Plant Cell.

The researchers found that in the laboratory research plant Arabidopsis, cauliflower mosaic virus recruits a protein complex called CDKC. This is the same protein complex that HIV uses, known in humans as P-TEFb. Since both viruses use this same process to trigger transcription, the scientists now know that this protein complex and its related genes have passed from species to species as organisms evolved over millions of years, Chen said.

"P-TEFb appears to be an evolutionarily conserved target of complex retro- and pararetroviruses for activating transcription," he said. "This must also reflect a fundamental mechanism for transcription inherited by these viruses."

Humans and organisms used for research, such as fruit flies and the tiny wormlike organism Caenorhabditis elegans, have only one gene in the protein complex that retroviruses use to activate transcription. These organisms die if that gene is completely blocked because of its essential role during transcription. This makes it difficult to analyze the function the gene may have in the organisms' growth, development and survival. Unlike those other organisms, the plant protein complex involves two genes.

"In Arabidopsis there are two genes for the CDKC protein complexes that trigger the transcription process," Chen said. "If we knock out one of these genes, the plants become resistant to CaMV and the plant is still growing."

The discovery of these two genes suggests that the mustard plant Arabidopsis is a better organism than others for studying how the proteins regulate gene function and transcription, he said.

However, blocking of one of the plant's genes caused some alteration of leaves, flowers and trichomes (tiny hairlike structures) and delayed flowering on the mutated plants, he said. In addition, mutant plants in which both genes were blocked died in the embryonic stage just as would an organism with only one gene.

Now that Chen knows that Arabidopsis has two genes involved in the transcription process, his research team wants to learn more about genes' possible roles in plant growth and development and where those tasks are performed. 

"The two genes each may have specialized functions depending on where they are activated in the plant," he said. "In some tissues the genes appear to be turned on in the same place. But, for example, in the flower, one gene is expressed in one particular place and the other gene is expressed in a different place."

The key question for researchers is how blocking the function of one protein inhibits transcription and replication of the viruses. Discovering the answer could mean major advances for prevention of retroviruses and treatment of the diseases they cause in plants and animals.

The other researchers on this study were postdoctoral research assistant Xiaofeng Cui and  research scientist Baofang Fan, both of the Purdue Department of Botany and Plant Pathology, and James Scholz, a University of Missouri Division of Plant Sciences professor.

Purdue's Agricultural Research Program provided funding for this project.

Writer:   Susan A. Steeves, (765) 496-7481,  ssteeves@purdue.edu

Source: Zhixiang Chen, (765) 494-4657, zhixiang@purdue.edu

Ag Communications: (765) 494-2722;
Beth Forbes, forbes@purdue.edu
Agriculture News Page

Note to Journalists: A copy of the study is available from Susan A. Steeves, (765) 496-7481, ssteeves@purdue.edu

PHOTO CAPTION:
Purdue molecular geneticist Zhixiang Chen is studying a plant virus that causes illness in the same way as human immunodeficiency virus (HIV). Cauliflower mosaic virus infected the laboratory plant Arabidopsis on the left. Blocking a gene in the plant on the right prevented infection. Chen's research eventually could lead to new treatments for the plant disease, HIV and other similar illnesses. (Purdue University photo/Alex Turco)

A publication-quality photo is available at http://news.uns.purdue.edu/images/+2007/chen-mosaicvirus.jpg

The C-terminal domain (CTD) of RNA polymerase II is phosphorylated during the transcription cycle by three cyclindependent kinases (CDKs): CDK7, CDK8, and CDK9. CDK9 and its interacting cyclin T partners belong to the positive transcription elongation factor b (P-TEFb) complexes, which phosphorylate the CTD to promote transcription elongation. We report that Arabidopsis thaliana CDK9-like proteins, CDKC;1 and CDKC;2, and their interacting cyclin T partners, CYCT1;4 and CYCT1;5, play important roles in infection with Cauliflower mosaic virus (CaMV). cdkc;2 and cyct1;5 knockout mutants are highly resistant and cdkc;2 cyct1;5 double mutants are extremely resistant to CaMV. The mutants respond normally to other types of plant viruses that do not replicate by reverse transcription. Expression of a reporter gene driven by the CaMV 35S promoter is markedly reduced in the cdkc;2 and cyct1;5 mutants, indicating that the kinase complexes are important for transcription from the viral promoter. Loss of function of CDKC;1/CDKC;2 or CYCT1;4/CYCT1;5 results in complete resistance to CaMV as well as altered leaf and flower growth, trichome development, and delayed flowering. These results establish Arabidopsis CDKC kinase complexes as important host targets of CaMV for transcriptional activation of viral genes and critical regulators of plant growth and development.